In solid structures, in particular in load-bearing structures of, for example, bridges, buildings, tunnels, railways, containment walls, dams, embankments, slabs and beams of buildings, underground piping and structures of urban subways, and the like, it is very important to monitor, in several points, significant parameters, particularly mechanical stresses (and thus those forces and/or pressures causing the stresses) to which the structure is subjected in those points. In the present description, solid structures are considered such as structures made from building material, for example cement, concrete, mortar.
Such a monitoring action, which is carried out either periodically or continuously, is useful both in an initial step and during the life time of the structure.
To this purpose, it is known the use of electronic monitoring devices based on electronic sensors, which are capable of providing good performances at low cost. Usually, these devices are applied to the surface of the structure to be monitored, or within recesses that are already provided in the structure and are accessible from the outside.
To enhance the monitoring performance, in view of an evaluation of the structure quality, safety, ageing, reaction to varying atmospheric conditions, and the like, approaches have been also carried out which provide electronic monitoring devices that are completely embedded, e.g. “buried” into the material (for example reinforced concrete) of which the structure to be monitored is made. Among those, a device is described in U.S. Pat. No. 6,950,767, which is actually a whole system encapsulated in one container, which includes several parts, assembled on a substrate, such as integrated circuits, sensors, antennas, capacitors, batteries, memories, control units and the like, which are fabricated in different “chips” that are connected to each other via electric connections provided with metal linkages.
As a whole, U.S. Pat. No. 6,950,767 describes an approach of the “System in Package” (SiP) type. However, it should be understood that a SiP, which is intended to be first “drowned” in a construction material (e.g. liquid concrete, which will then solidify) and then remain “buried” within the solid structure, is subjected to adverse conditions, for example due to very high pressures, which can even be as high as several hundreds of atmospheres. In addition, a number of other causes of wear exist, over time, for example, due to water seepages, which can damage the system.
In view of the above, the level of reliability of a SiP such as the one described in U.S. Pat. No. 6,950,767 may not be fully satisfactory.
A further approach is represented by devices such as those described in the international patent application WO 2012/084295, of the present Assignee. WO 2012/084295 describes a device for the detection and monitoring of one or more local parameters within a solid structure, comprising an integrated detection module, in which at least one integrated sensor and the circuitry thereof, as well as an electromagnetic unit, for communication to and from the outside, are provided. The integrated detection module further comprises a passivation layer which completely covers the integrated sensor and the circuitry thereof, such that the integrated detection module is completely hermetically sealed and galvanically insulated from the surrounding environment.
The device in WO 2012/084295 ensures the required functionality with a satisfactory level of reliability and robustness relative to the above-mentioned causes of wear. It can therefore be used for detecting a mechanical stress and/or a force and/or a pressure within the solid structure to be monitored, at the point where it is positioned. With reference to the detection of mechanical stress, the device in WO 2012/084295 employs sensors of pressure (thus, also of force, and also of mechanical stress) which operate based on the piezo-resistivity phenomenon.
As known, and as will be better illustrated herein below, the piezo-resistivity, i.e. piezo-resistive effect, defines a dependence between an electric signal generated by a piezo-resistive sensor and a mechanical stress (i.e., strain, i.e., compression or tension) experienced by the sensor material (e.g., silicon). The mechanical stress can be, in turn, representative of a pressure and/or force to which the sensor is subjected. The sensitivity, intended as the ratio of the force applied to the electric signal generated, depends on the reaction of the material (silicon) to the stresses, which, in turn, depends on the crystal orientation of the silicon. The sensitivity is thus a function of the direction along which the force, and the consequent mechanical stress, are applied.
An example of a diagram of angular sensitivity (piezo-resistive coefficient of the silicon) is illustrated in FIG. 1A. This figure will be described in greater detail below. For the present purpose of outlining the technical problem addressed by the present embodiments, it should be simply assumed that the vertical axis of FIG. 1A, referred to as [001], coincides with the vertical axis, which is properly intended as the direction along which the force of gravity acts. This corresponds to orientating the sensor, based on the crystal orientation thereof, so that a main sensitivity lobe is aligned with the direction of action of the force of gravity. Thereby, the mechanical stress corresponding to the force applied by the weight of the structure, in the monitoring point, can be detected with good approximation. This is useful, because, usually, the weight of the structure is the more relevant force in play, and is often also the one that is desired to be detected.
However, what is actually measured by a sensor, such as that in WO 2012/084295, does not correspond exactly to the vertical component of the force, but is also affected by any “horizontal” component, i.e., a component acting on a “horizontal” plane, orthogonal to said vertical direction. From FIG. 1A, which substantially shows a section along a plane orthogonal to the “horizontal” one, it can be observed that sensitivity lobes also exist in the direction [010].
More specifically, FIG. 1B shows the sensitivity lobes on the “horizontal” plane, orthogonal to the sectional plane in FIG. 1A, in the directions [100] and [010]. As a whole, FIGS. 1A and 1B illustrate the “three-dimensional” dependence of the sensitivity on mechanical stress (compression or tension) experienced by the sensor, as a function of a force and/or pressure that is applied in any direction. This dependence may be also figured as a three-dimensional diagram, in which the volumetric lobes are shown as being similar to rotational ellipsoids.
Now, the overall intensity as measured by prior art devices, such as those mentioned above, comprises a contribution deriving from the vertical component of the force (towards which the main sensitivity lobe is orientated in the direction [001]), but also a contribution deriving from the horizontal (or “lateral”, in an equivalent definition) component of the force, on the plane orthogonal to the direction [001], depending on the angle of application of such a force relative to the crystal orientation, as shown in FIG. 1B. Therefore, the prior art sensors measure the resulting force at the point where the sensor is positioned.
If it is desired to detect the vertical component of the force, and in case this component (weight force) is much greater than the lateral component, the intensity, as detected by the sensors of the prior art devices, is a good approximation of the result that is desired to be achieved. However, the following drawbacks will usually arise: i) as stated above, the result of the sensor measurement does not correspond to the vertical component; ii) the result of the sensor measurement may not correspond even to the actual intensity of the force, as the two vertical and transversal components may result as being weighted differently by different sensitivity values; iii) is not possible to differentiate the two components, nor measure them separately; and iv) in case the lateral force is desired to be measured, this cannot be done by simply orientating the sensor so as to align the direction of maximum sensitivity with one of the axes or [100]. In fact, in this case, the result would be influenced also by the component along the axis [001], which in this example is aligned with the vertical direction, and thus is affected by the weight of the structure, which may totally impair the correctness and accuracy of the result.
With particular reference to the drawback iv), there emerges an aspect related to the requirement of also measuring the lateral forces. In fact, generally, the building structures to be monitored may be anisotropic systems, wherein each point may be subjected to forces/stresses in different directions, i.e., to forces/stresses having at least two components which are desired to be measured separately. By way of example, a load bearing column of a structure, or a bridge pillar can be mentioned.
Particularly, the designers and the maintenance operators of such a type of solid structure need to know also the lateral mechanical stresses (flexural, compression or tension stresses) that are present in those points to be monitored, which are due for example to wind or to particular structural configurations. The information deriving from the knowledge also of the lateral stresses may be crucial for effective monitoring and maintenance of the solid structure to be monitored. From the above, it should be understood that a desire may be strongly felt to improve the accuracy and correctness of detection, and particularly to allow individual detections of both the lateral and the vertical components of the mechanical stress, or in other words, to detect the component of the force applied to a point in a specific direction of interest (i.e., in a given direction). This desire, due to the above reasons, may not met by the prior art devices mentioned above.